An adjustable pitch propeller for a watercraft can comprise a hub and a plurality of blades which are fixed on the hub but which allows adjustment of each blade about a blade axis to permit selection of the pitch thereof.
In the German patent document DE-AS 19 21 337, such a propeller is described in which each blade has a disk-shaped base which is centered in a seat of the hub, the angular position of the blade with respect to an axis thereof being selected by the engagement of a positioning pin extending downwardly from the seat in a selected registering hull of the blade base.
To allow selection of the angular position of the blade about the centering pin or boss and thus about the respective blade axis, the blade base has a multiplicity of registering bores, generally distributed in concentric circles or segments of a circle. Each registering bore can be brought into registry with a bore or hull of the seat so that a positioning pin can be inserted into the aligned or registered bores to thereby fix the pitch of the particular blade. The fastening of the blade base to the hub can then be effected by screws or bolts which can pass through elongated holes or slots in the blade base and can be threaded into internally threaded bores of the hub or seat.
Ship propellers of this type generally have only a limited possibility for adjustment of the blade pitch to accommodate different operating conditions.
However, for many types of sports watercraft, yachts and the like, hereinafter referred to as sport boats, utilizing a motor drive, it is found that a specific propeller pitch must be matched with the drive motor. This can pose a significant problem, especially where the propeller has only a limited capacity for pitch adjustment. In fact, it is found that the pitch of the propeller can have a significant effect on the engine speed so that, for example, a pitch change of about 1 mm which can correspond to a change in the angular orientation of each blade by about 1.degree., can result in a change in the engine speed of up to 200 revolutions per minute (RPM).
If one wishes, therefore, to design a standardized boat propeller which can be employed for a wide variety of engine speeds and sport boat requirements, utilizing a hub and propeller blades angularly adjustable on respective seats of the hub about respective axes transverse to the rotation axis of the hub, it is found that each blade base and the respective seat of the hub must be provided with about 30 registering bores to allow coverage of all of the possible desirable blade pitches for the propeller.
Furthermore, because of the large number of registration bores there is always the danger that an incorrect bore will be selected for the pitch adjustment and thus that the blades of the propeller might not be properly matched with one another or an incorrect pitch might be selected. Furthermore, fabrication of the blades is rendered complex because of the large number of bores which may be necessary and the requirement that they be precisely located on the propeller base. In many instances, the blade base simply cannot accommodate the large number of bores which may be required.